[Info-vax] VAX vs. MV/8000 [was Re: Hard links on VMS ODS5 disks]

[Arne Vajhøj]

On 8/26/2023 9:34 AM, Dan Cross wrote:
> In article <ucbh82$8kpo$2 at dont-email.me>,
> Arne Vajhøj  <arne at vajhoej.dk> wrote:
>> [snip]
>> DEC chose to have the VAX switch mode between 32 and
>> 16 bit instructions (similar to what AMD did a couple
>> of decades later).
> 
> One presumes you are referring to amd64 here.  If so, then nope,
> amd64 is an extension to 32-bit x86, not a replacement in the
> sense that the VAX ISA replaced the PDP-11 ISA when the mode bit
> was set.
> 
> Even in 64-bit mode, many (most?) programs will continue to
> operate as expected with 32- and even many 16- bit instructions.
> The same was not true of the VAX.

x86-64 in long mode has an emulation mode set via
an emulation bit that allows it to execute 32 bit
programs.

That is the same as what VAX did for PDP-11
compatibility.

The rule is that one cannot mix 64 bit and 32 bit
code in the same process.

Most 32 bit instructions actually work identical
in 64 bit mode, but some produce different results
and some causes a crash.

So executing 32 bit code not in compatibility mode
is a big no no.

Arne